1. Field of the Invention
The present invention relates to an optical pickup and an optical disk drive apparatus used to record and/or reproduce an information signal on and/or from an optical disk as an optical recording medium, by irradiating the optical disk with laser light.
2. Description of the Related Art
FIG. 8 shows a brief configuration of an optical disk drive apparatus 100 according to a related art. Here, reference numeral 1 denotes an optical disk, 2 denotes an optical pickup, and 3 denotes a spindle motor that rotationally drives the optical disk 1. The optical pickup 2 includes a laser source 6, a collimator lens 7, a beam splitter 8, a condensing lens 9, a photodetector 10, an objective lens 20, and an actuator 11 for performing focusing control and tracking control of the objective lens 20.
The optical disk 1 includes a substrate 12, an information signal recording layer 13, and a cover layer 14. The information signal recording layer 13 is formed on the substrate 12, and is made of a phase change material in which a phase condition is reversibly changeable. The cover layer 14 is made of a transparent resin material and has a thickness of about 0.1 mm. The information signal recording layer 13 has a spiral recording track or a concentric recording track. The information signal recording layer 13 may be made of a magneto-optical recording material, or a metal reflection film having pits (irregularities) formed therein. The optical pickup 2 is arranged to face the cover layer 14 of the optical disk 1.
FIG. 6 shows a configuration of the actuator 11. The actuator 11 includes a fixed portion 15 and a movable portion 16. The fixed portion 15 includes permanent magnets 17a and 17b, a yoke 18, and a support base 19. The movable portion 16 includes the objective lens 20, a focusing coil 21, and a tracking coil 22, as well as a lens holding member 23 that holds these components. Elastic supporting members 24a, 24b, 24c, and 24d are linear, elastic, and highly conductive, first ends of which are fixed to the support base 19, and second ends of which are retained at the movable portion 16. The elastic supporting members 24a, 24b, 24c, and 24d retain the movable portion 16, such that the movable portion 16 is movable in a direction perpendicular to the optical disk 1, and in a radial direction of the optical disk 1. Also, the elastic supporting members 24a, 24b, 24c, and 24d are electrically connected with the focusing coil 21 and the tracking coil 22 on the lens holding member 23.
Referring back to FIG. 8, the optical disk drive apparatus 100 also includes an error signal generating circuit 4 and a control circuit 5 for performing the focusing control and the tracking control of the actuator 11. The control circuit 5 supplies the focusing coil 21 and the tracking coil 22 with control current through the elastic supporting members 24a, 24b, 24c, and 24d (shown in FIG. 6).
To record an information signal, the optical disk 1 is rotationally driven by the spindle motor 3. In this state, laser light emitted from the laser source 6 and modulated into a pulse form is collimated by the collimator lens 7. The beam splitter 8 transmits the collimated laser light, and the objective lens 20 converges the laser light, so as to be condensed into a very small light spot on the information signal recording layer 13 through the cover layer 14 of the optical disk 1.
The information signal recording layer 13 of the optical disk 1 is repeatedly heated or cooled through irradiation of pulse-modulated laser light. Because of the difference between the processes, a recording mark whose phase condition is changed into an amorphous phase or a crystal phase is formed as an information signal.
To reproduce a recorded information signal, the optical disk 1 is rotationally driven by the spindle motor 3, similarly. In this state, the optical disk 1 is irradiated with laser light, with a constant intensity emitted from the laser source 6, through the cover layer 14 of the optical disk 1, so as to be condensed into a very small light spot on the information signal recording layer 13. The intensity of reflection light from the information signal recording layer 13 at this time varies in accordance with a recording mark, and an information signal is thus reproduced.
During a recording or reproducing operation of the information signal, the light beam reflected by the optical disk 1 is reflected by the beam splitter 8, condensed by the condensing lens 9, and detected by the photodetector 10. The photodetector 10 has a plurality of divided photo-detecting surfaces. The error signal generating circuit 4 generates a focusing error signal and a tracking error signal on the basis of a detection signal from each photo-detecting surface.
The control circuit 5 supplies the focusing coil 21 and the tracking coil 22 with control current based on the focusing error signal and the tracking error signal, through the elastic supporting members 24a, 24b, 24c, and 24d. The actuator 11 drives the movable portion 16 in a direction perpendicularly toward or away from the optical disk 1, and in a radial direction orthogonal to the recording track, by using an electromagnetic force generated between the control current and magnetic flux caused by the permanent magnets 17a and 17b. 
Even when the optical disk 1 is perpendicularly displaced due to face deflection, the focusing control is performed so that the laser light is accurately condensed into the light spot on the recording track, to follow the displacement. Also, even when the recording track is displaced in the radial direction due to eccentricity, the tracking control is performed so that the laser light scans the recording track to follow the displacement.
An operating distance (clearance between the objective lens 20 and the surface of the cover layer 14) tends to be decreased as the numerical aperture (NA) of the objective lens 20 is increased. In addition, reduction in size of the objective lens 20 is desired in association with a demand for a reduction in size of the optical disk drive apparatus. The operating distance, which has been 0.8 mm or larger, is desired to be reduced to 0.2 to 0.3 mm.
When the actuator 11 normally performs the focusing control, the operating distance is constantly kept between the objective lens 20 and the surface of the cover layer 14. However, in some cases, the control of the actuator 11 is not performed normally, for example, when an impact or a vibration is applied to the apparatus from the outside, in a non-operating state, in which the control of the actuator 11 is suspended. Also, in some cases, a normal error signal is not obtained at the start of the control operation or during the control operation, when being affected by an impact or a vibration applied from the outside, or due to contamination on, or damage to, the optical disk 1. In such a case, the movable portion 16 may be excessively displaced, and approach the optical disk 1. Ultimately, the movable portion 16 may contact the optical disk 1. Regarding actual assembly accuracy of the apparatus, it is difficult to provide a mechanism, such as a stopper, for regulating the excessive displacement of the movable portion 16 at an intermediate position of the operating distance.
Thus, it is difficult to always completely prevent the contact of the movable portion with the optical disk 1, as a result of the excessive displacement of the movable portion 16. Even when the contact occurs, it is at least necessary to take a countermeasure to prevent the direct contact between the objective lens 20 and the optical disk 1, so as to prevent the objective lens 20 and the optical disk 1 from being damaged. Otherwise, recording or reproducing performance of an information signal deteriorates significantly.
As such a countermeasure, Japanese Patent Laid-Open No. 10-320802 discloses an example in which a ring-like protection member is provided at the periphery of an objective lens to protrude toward an optical disk from the objective lens.
FIG. 7 is a cross-sectional view showing a configuration of an objective lens holding portion described in the above publication. An objective lens 20 is mounted in a mounting hole 26 formed at a lens holding member 23, such that a lower surface 25b of an edge portion 25 contacts a bearing surface formed at the periphery of the mounting hole 26. Also, a protection member 27 is mounted on an upper surface 25a of the edge portion 25.
The protection member 27 is made of a soft resin material, an upper surface of which is higher than at least a vertex of a curved surface of the objective lens 20, and is at a height less than the operating distance. Hence, in a normal control operation, the protection member 27 would not contact the optical disk 1. When a movable portion is excessively displaced, and approaches the optical disk 1 in a condition other than a normal operation, although the protection member 27 contacts the optical disk 1, the objective lens 20 is protected without directly contacting the optical disk 1.
In such a case, as the frequency of contact between the protection member and the optical disk increases, the protection member is subjected to wear or is damaged, and would lose its protection function for the objective lens with time. The surface of the cover layer of the optical disk may be damaged similarly, and hence normal recording and/or reproducing of an information signal may become difficult. To avoid this, and to allow the protection member to be used for a long term, the height of the protection member should be minimized, to fall in a range of the operating distance of the objective lens, to reliably protect the objective lens. Meanwhile, a sufficient clearance should be provided between the protection member and the optical disk, to minimize the frequency of contact with the cover layer.
As described above, a protruding distance of the protection member from the objective lens is necessary to be within a predetermined range. If the protruding distance is above the upper limit, the frequency of contact between the protection member and the optical disk is increased, and hence, damage of not only the protection member, but also, the optical disk, may be expected.
In contrast, if the protruding distance is below the lower limit, the objective lens may directly contact the optical disk as a result of wear of the protection member, and an adhering substance on the optical disk may contact the objective lens.
For example, when an objective lens has an NA of about 0.65, and an operating distance of 0.8 mm or larger, the objective lens can be protected, as long as a protruding distance of a protection member from the objective lens is within a relatively wide range (for example, within a range of from 0.05 to 0.65 mm). Also, a sufficient clearance can be provided between the protection member and the optical disk.
For another example, when an objective lens has an NA of about 0.85, and an operating distance of 0.3 mm, a protruding distance of a protection member from the objective lens has to be within a relatively narrow range, for example, within a range of from 0.05 to 0.15 mm.
In this way, as the NA is increased due to an increase in density of the optical disk, the protruding distance of the protection member from the objective lens has to be within the narrow range.
However, the extremely thin, sheet-like protection member as in the related art is soft, and its intensity is not sufficient. Hence, the surface of the protection member is uneven, because the protection member is deformed when being bonded, resulting in the height thereof being uneven. The protection member and the adhesive have uneven thicknesses. Therefore, it has been extremely difficult to bring the height of the protection member within the narrow range.